arXiv subjects

K. W. West

Publications and source records attributed to K. W. West.

At least 361 records · Page 20Linked to original sources

arXiv2004

Interaction Correction to the Longitudinal Conductivity and Hall Resistivity in High Quality Two-Dimensional GaAs Electron and Hole Systems

We present a systematic study of the corrections to both the longitudinal conductivity and Hall resistivity due to electron-electron interactions in high quality GaAs systems using the recent theory of Zala et al. [Phys. Rev. B 64, 214204 (2001)]. We demonstrate that the interaction corrections to the longitudinal conductivity and Hall resistivity predicted by the theory are consistent with each other. This suggests that the anomalous metallic drop in resistivity at B=0 is due to interaction effects and supports the theory of Zala et al.

cond-mat.str-el↗

arXiv2004

Possible observation of phase coexistence of the $ν=1/3$ fractional quantum Hall liquid and a solid

We have measured the magnetoresistance of a very low density and an extremely high quality two-dimensional hole system. With increasing magnetic field applied perpendicularly to the sample we observe the sequence of insulating, $ν=1/3$ fractional quantum Hall liquid, and insulating phases. In both of the insulating phases in the vicinity of the $ν=1/3$ filling the magnetoresistance has an unexpected oscillatory behavior with the magnetic field. These oscillations are not of the Shubnikov-de Haas type and cannot be explained by spin effects. They are most likely the consequence of the formation of a new electronic phase which is intermediate between the correlated Hall liquid and a disorder pinned solid.

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arXiv2004

Cascade of Quantum Phase Transitions in Tunnel-Coupled Edge States

We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields $B_{c}$ is a function of a single scaling argument $|B-B_{c}|T^{-κ}$, where the exponent $κ= 0.42$. This puzzling resemblance to a quantum Hall-insulator transition points to importance of interedge correlation between the coupled edge states.

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arXiv2004

Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System

At total Landau level filling factor $ν_{tot}=1$ a double layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence. This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers. In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low temperature limit. This finding supports the prediction that the ground state at $ν_{tot}=1$ is an excitonic superfluid.

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arXiv2004

Evidence of a first order phase transition between Wigner crystal and Bubble Phases of 2D electrons in higher Landau levels

For filling factors $ν$ in the range between 4.16 and 4.28, we simultaneously detect {\it two} resonances in the real diagonal microwave conductivity of a two--dimensional electron system (2DES) at low temperature $T \approx 35$ mK. We attribute the resonances to Wigner crystal and Bubble phases of the 2DES in higher Landau Levels. For $ν$ below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first order phase transition. We estimate the transition point as $ν=4.22$.

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arXiv2003

Metastable Resistance Anisotropy Orientation of Two-Dimensional Electrons in High Landau Levels

In half-filled high Landau levels, two-dimensional electron systems possess collective phases which exhibit a strongly anisotropic resistivity tensor. A weak, but as yet unknown, rotational symmetry-breaking potential native to the host semiconductor structure is necessary to orient these phases in macroscopic samples. Making use of the known external symmetry-breaking effect of an in-plane magnetic field, we find that the native potential can have two orthogonal local minima. It is possible to initialize the system in the higher minimum and then observe its relaxation toward equilibrium.

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arXiv2003

Ultrafast Dynamic Control of Spin and Charge Density Oscillations in a GaAs Quantum Well

We use subpicosecond laser pulses to generate and monitor in real time collective oscillations of electrons in a modulation-doped GaAs quantum well. The observed frequencies match those of intersubband spin- and charge-density excitations. Light couples to coherent density fluctuations through resonant stimulated Raman scattering. Because the spin- and charge-related modes obey different selection rules and resonant behavior, the amplitudes of the corresponding oscillations can be independently controlled by using shaped pulses of the proper polarization.

cond-mat↗

arXiv2003

Light Scattering by Low Lying Quasiparticle Excitations in the Fractional Quantum Hall Regime

Low lying excitations of electron liquids in the fractional quantum Hall (FQH) regime are studied by resonant inelastic light scattering methods. We present here results from charge and spin excitations of FQH states in the lowest spin-split Landau levels that are of current interest. In the range of filling factors $2/5 \geq ν\geq 1/3$, we find evidence that low energy quasiparticle excitations can be interpreted with spin-split composite fermion quasi-Landau levels. At FQH states around $ν=3/2$, we find well-defined excitations at 4/3 and 8/5 that are consistent with a spin-unpolarized population of quasi-Landau levels.

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arXiv2003

Evidence for current flow anomalies in the irradiated 2D electron system at small magnetic fields

We report experimental results of low temperature magnetotransport in high mobility 2D electron systems exposed to radiation up to 20GHz frequency using a simple dipole configuration. Magnetoresistance oscillations are observed as in previously reported higher frequency radiation on 2D systems, however minima here can be seen to extend to negative biases, and zeroes previously reported are not observed persistently around the full sample perimeters. In addition, under radiation, voltages are observed from internal to external contacts in the absence of applied driving currents not due to simple rectification. These findings are consistent with micro- and macroscopic theoretical pictures of radiation induced transport and current instabilities due to local negative resistivities. However, the temperature dependent development of minima is shown to be severely power dependent, and outside of present theoretical understanding.

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arXiv2003

Fermi liquid behavior of metallic 2D holes at high temperatures

The resistivity $ρ$ of high mobility dilute 2D holes in GaAs exhibits a peak at a certain temperature $T^*$ in zero magnetic field($B$=0). In the $T>T^*$ regime where d$ρ$/d$T<$0, we observe for the first time both the $ν=1$ quantum Hall(QH) effect and a low field insulator-QH transition which is consistent with the 2D hole system being a Fermi liquid(FL). The known linear $T$-dependent conductivity in this $T$ regime then can be explained by hole-hole Coulomb interactions of this FL. The fact that the system is metallic (d$ρ$/d$T>$0) for $T<T^*$ implies that the high temperature FL transforms into the 2D metallic state in the neighborhood of $T^*$.

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arXiv2003

Wigner crystalization about $ν$=3

We measure a resonance in the frequency dependence of the real diagonal conductivity, Re[$σ_{xx}$], near integer filling factor, $ν=3$. This resonance depends strongly on $ν$, with peak frequency $f_{pk} \approx 1.7$ GHz at $ν=3.04$ or 2.92 close to integer $ν$, but $f_{pk} \approx$ 600 MHz at $ν=3.19$ or 2.82, the extremes of where the resonance is visible. The dependence of $f_{pk}$ upon $n^*$, the density of electrons in the partially filled level, is discussed and compared with similar measurments by Chen {\it et al.}\cite{yong} about $ν=1$ and 2. We interpret the resonance as due to a pinned Wigner crystal phase with density $n^*$ about the $ν=3$ state.

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arXiv2003

Observation of narrow-band noise accompanying the breakdown of insulating states in high Landau levels

Recent magnetotransport experiments on high mobility two-dimensional electron systems have revealed many-body electron states unique to high Landau levels. Among these are re-entrant integer quantum Hall states which undergo sharp transitions to conduction above some threshold field. Here we report that these transitions are often accompanied by narrow- and broad-band noise with frequencies which are strongly dependent on the magnitude of the applied dc current.

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arXiv2003

Experiments on the Fermi to Tomonaga-Luttinger liquid transition in quasi-1D systems

We present experimental results on the tunneling into the edge of a two dimensional electron gas (2DEG) obtained with GaAs/AlGaAs cleaved edge overgrown structures. The electronic properties of the edge of these systems can be described by a one-dimensional chiral Tomonaga-Luttinger liquid when the filling factor of the 2DEG is very small. Here we focus on the region where the Tomonaga-Luttinger liquid breaks down to form a standard Fermi liquid close to $ν=1$ and show that we recover a universal curve, which describes all existing data.

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arXiv2003

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range $2/5 \geq ν\geq 1/3$. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor $ν\gtrsim 1/3$ and at $ν\lesssim 2/5$ identify a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At $ν= 1/3$ energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.

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arXiv2003

Crossover and Coexistence of Quasiparticle Excitations in the Fractional Quantum Hall Regime at $ν\leq 1/3$

New low-lying excitations are observed by inelastic light scattering at filling factors $ν=p/(ϕp \pm 1)$ of the fractional quantum Hall regime with $ϕ=4$. Coexisting with these modes throughout the range $ν\leq 1/3$ are $ϕ=2$ excitations seen at 1/3. Both $ϕ=2$ and $ϕ=4$ excitations have distinct behaviors with temperature and filling factor. The abrupt first appearance of the new modes in the low energy excitation spectrum at $ν\lesssim 1/3$ suggests a marked change in the quantum ground state on crossing the $ϕ=2 \to ϕ=4$ boundary at $ν= 1/3$.

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arXiv2003

Observation of Apparently Zero-Conductance States in Corbino Samples

Using Corbino samples we have observed oscillatory conductance in a high-mobility two-dimensional electron system subjected to crossed microwave and magnetic fields. On the strongest of the oscillation minima the conductance is found to be vanishingly small, possibly indicating an insulating state associated with these minima.

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arXiv2003

Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor

At low Landau level filling of a two-dimensional electron system, typically associated with the formation of an electron crystal, we observe local minima in Rxx at filling factors nu=2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9. Each of these developing fractional quantum Hall (FQHE) states appears only above a filling factor-specific temperature. This can be interpreted as the melting of an electron crystal and subsequent FQHE liquid formation. The observed sequence of FQHE states follow the series of composite fermion states emanating from nu=1/6 and nu=1/8.

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arXiv2003

Fractional Quantum Hall Effect of Composite Fermions

In a GaAs/AlGaAs quantum well of electron density 1x10^{11} cm^{-2} we observe a fractional quantum Hall effect (FQHE) at filling factors nu=4/11, and 5/13, and weaker states at nu=6/17, 4/13, 5/17 and 7/11. These sequences of fractions do not fit into the standard series of integral quantum Hall effects (IQHE) of composite fermions (CF) at nu=p/(2mp +/- 1). They rather can be regarded as the FQHE of CFs attesting to residual interactions between these composite particles. In tilted magnetic fields the nu=4/11 state remains unchanged, strongly suggesting it to be spin-polarized. The weak nu=7/11 state vanishes quickly with tilt.

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